1. Field of the Invention
The present invention relates to a catalyst for producing a polymer of a vinyl compound (including .alpha.-olefin polymers and aromatic compound polymers), and a process for producing a vinyl compound polymer employing the catalyst. More particularly, the present invention relates to a process for producing an aromatic vinyl compound polymer having mainly syndiotactic structure with high catalyst activity and with high selectivity.
2. Description of the Related Art
Aromatic Vinyl compound polymers include three structure types of polymers: syndiotactic polymers, isotactic polymers, and atactic polymers.
Of these, the polymers of the syndiotactic structure, which have a high melting point and crystallize quickly in comparison with polymers of other structures, are useful as heat-resistant polymers. The syndiotactic aromatic vinyl compound polymer is produced, for example, in the presence of a catalyst formed by contact of a titanium compound such as titanium halide and alkoxytitanium with an organoaluminum compound such as methylalumoxane as disclosed in Japanese Patent Application Laid-Open No. 62-04818.
However, in the polymerization of styrene monomer with a catalyst system constituted of a combination of a titanium compound such as titanium tetrachloride and tetraethoxytitanium with methylalumoxane, the catalyst activity is low, and the catalyst remains in the formed polymer in a considerable amount. Therefore the polymer is presumed to discolors significantly during high-temperature molding, and not to be suitable for practical use.
On the other hand, a catalyst system composed of methylalumaxane and complex obtained by reaction of a transition metal compound, like titanium tetrachloride, with an organic compound, like 2.2'-dihydroxy-3.3'-di-tert-butyl-5.5'-dimethyl-diphenylsulbide gives a slightly lower content of stereoregular polymer owing chiefly to atactic polymer formation as a by-product even though the catalyst exhibits considerably high catalytic activity. The amorphous polymer coexisting in a larger amount affects adversely the melting point and the crystallization velocity of the polymer. Therefore, removal of the amorphous polymer is required by solvent extraction or the like treatment, disadvantageously.
After comprehensive investigation, it was found by the present inventors that a specific organometal complex in combination of methylalumoxane enables production of aromatic vinyl compound polymers of syndiotactic structure with high catalyst activity and high selectivity, and the present invention has been completed.